Active implantable medical devices such as those defined in directive 90/385/EEC of Jun. 20, 1990 of the European Community Council are known to analyse different signals emitted by the myocardium, which signals are composed of different waves, to detect the spontaneous depolarisation of the myocardium. Such devices include cardiac pacemakers, defibrillators and/or cardiovertors. Detection of a depolarisation wave in these signals is revealing of spontaneous myocardial cell activity, either in the atrial cavity or the ventricular cavity, and is necessary for the various functions of these devices.
The detection of such a depolarisation wave allows, first of all, to reset various temporal (i.e., timing related) commands of the device, such as an escape-interval, or, to release (i.e., trigger) some other function, such as an atrio-ventricular delay.
This detection is also used to command the functioning of certain algorithms conventionally implemented in the implanted device, such as algorithms of fallback, rate smoothing, etc. The detection of the spontaneous ventricular rhythm, particularly the analysis of its stability, is also, in some implantable defibrillators, an essential parameter for the appropriate release of a shock therapy.
Finally, some devices are equipped with "Holter" memory functions allowing for continuous monitoring of parameters of the cardiac rhythm, especially for evaluating the stability of the rhythm for diagnostic and/or therapeutic purposes.
Systems for the detection of depolarisation waves are known and they use various means. One such system is described in EP-A-0 605 264 and its corresponding U.S. Pat. No. 5,388,586, commonly owned by the assignee of this application Ela Medical, which uses a filtering of the myocardial signal in a selected frequency range, for example 30 to 80 Hz, where the filtered signal is then applied to a threshold circuit, with unilateral or bilateral threshold, for detection of the depolarisation wave.
More elaborate systems are also known, for example, the system discussed in EP-A-0775 502 and its corresponding U.S. patent application Ser. No. 08/755,335, now U.S. Pat. No. 5,836,980 also commonly owned by Ela Medical, where detection is based on an analysis of the first derivative (rate of increase) of the signal.
In all cases, the detection of the presence of a depolarisation wave in the most rapid and reliable (with regard to sensitivity) manner possible is desired. Indeed, the rapidity of detection is important because the presence of a possible spontaneous depolarisation wave must be signalled before the end of the relevant escape interval, otherwise stimulation could be applied unnecessarily.
Although not harmful, it is desirable to avoid such a stimulation simultaneous to spontaneous activity (a phenomenon known as "fusion"), on the one hand to save energy, and on the other hand, so that the spontaneous activity of the patient can be monitored by the system without the presence of such extraneous signals (i.e., the stimulation), for example, for diagnostic functions or for algorithms which are able to predict arrhythmia based on the stability of the spontaneous rhythm.
Known detection processes suitably meet sensitivity and rapidity requirements in the detection of the presence of a spontaneous depolarisation wave. Nevertheless, they suffer the disadvantage of not always indicating, from one cycle to the next, the same instant at which the depolarisation wave is detected.
As the depolarisation wave comprises several phases that may vary slightly from one cycle to another and one can find in some cycles, a slight modification or variation of amplitude, or even morphology, prior art detection systems can only detect alternately one phase of the wave or another (for example, a positive peak or a negative peak). As a result, a stable rhythm (in regard of the different components of the signal) will appear unstable to those systems, with variations which are typically greater than 20 ms.
This presents a notable disadvantage precisely when the objective is to measure temporal parameters of the rhythm, for example, for purposes of diagnosis or to determine the behaviour of certain algorithms.